In some electrical circuit packaging designs, it is desirable to provide a separable electrical connection between a single or multi-chip module substrate and the circuit board on which it is mounted. Such an electrical connection is generally considered "separable" if the chip module substrate can be easily unplugged from the circuit board and subsequently attached to another circuit board. The need for such separable connections is due to economies, in that it is cost-effective to use the same chip module substrates on a variety of circuit boards.
A conventional socket connector system is illustrated in FIG. 1. A first circuit substrate 10 and a second circuit substrate 12 are provided. A dendrite interposer 14 is disposed between first circuit substrate 10 and second circuit substrate 12. Dendrite interposer 14 is typically on the order of several millimeters thick. The Flexiposer.TM. interposer by International Business Machines Corporation is one such type of dendrite interposer 14. Dendrite interposer 14 includes: (a) a carrier 15 made of polyamide film or other similar type of circuit board material, and (b) pairs of electrical contact pads 16a and 16b attached to carrier 15. For a given pair, contact pads 16a and 16b are attached to carrier 15 on opposing sides and electrically coupled to one another through vias 18 formed in carrier 15. On a given contact pad 16a or 16b, a plurality of ridges 26 extend out from contact pads 16a and 16b toward one of the circuit substrates 10 or 12. Ridges 26 are typically needle-like structures of palladium which are grown off circuit contact pad 16a or 16b. Ridges 26 generally function to penetrate dust and debris in forming an electrical contact between mating surfaces of circuit substrates 10 and 12. Ridges 26 are typically on the order of 0.025 to 0.051 mm (1 to 2 mils) in height.
A solder body 20 is disposed between first circuit substrate 10 and electrical contact pad 16a attached to dendrite interposer 14. Mounting end 30 of solder body 20 is mounted to first circuit substrate 10. Contact end 32 of solder body 20 faces contact pad 16a. Compressive forces are exerted, in the direction of arrows A of FIG. 1, on first circuit substrate 10 and second circuit substrate 12 to force all areas of contact ends 32 to engage and form reliable electrical contacts with all of the contact pads 16a (see area 22 of FIG. 1).
According to the conventional socket connector system illustrated in FIG. 1, however, rarely do all of the solder bodies 20 form reliable electrical contacts with the contact pads 16a. Often, solder body 20 makes only a poor or non-existent electrical contact with contact pad 16a (see area 24 of FIG. 1). This results from at least two inherent tolerance variations in the conventional socket connector system of FIG. 1. First, there are often variations in height between the two circuit substrates 10 and 12 to be mated due to inconsistencies in the flatness of one or both of the circuit substrates 10 or 12. These variations may also arise due to inconsistencies in the thickness of any hardware located between the circuit substrates 10 and 12. On a typical system, where two circuit substrates 10 and 12 are to be mated, a variation in height between approximately 0.05 to 250 mm (2 to 10,000 mils) may exist between electrical contact pads 16a and solder bodies 20. Second, the rigid nature of the solder material in solder body 20 renders it unlikely that all of the solder bodies 20 throughout the system will deform to the extent necessary under typical compressive forces to provide uniformly the desired reliable electrical connections.
Accordingly, a need remains for an improved socket connector system which forms a separable and reliable electrical connection between circuit substrates.